Alkyd resins are commonly used in coating systems. Alkyd resins may be formed through the incorporation of unsaturated fatty acid esters into polyester or polyurethane chain-extended polymer systems. The simplest alkyd polyesters are those based on reaction products of tri-glycerides of unsaturated fatty acids.
In early resins of this type used in coating applications, speed of cure (drying speed) was increased through thermal advancement ("cooking") of the resin and viscosity was reduced by incorporation of a solvent prior to use. Such resins comprise fatty acid tri-glycerides which have undergone some degree of coupling of their linkages to generate higher molecular weight, cross-linked systems. Generally speaking, this cross-linking is accomplished thermally at high temperatures through free radical addition reactions or oxidatively. The latter process is promoted by incorporation of transition metal complexes capable of catalyzing the auto-oxidation of the allylic hydrogens associated with the unsaturated fatty acids to generate hydroperoxides and peroxides which are capable of facilitating cross-link formation between fatty acid double bonds by initiating free radical addition processes.
Later versions of alkyds are based on transesterified tri-glycerides. In these systems, a tri-glyceride is reacted with excess glycerol to generate a mixture of mono-, di-, and tri-glycerides, with the mono-glyceride component present in the largest amount. The resulting pre-polymer then is reacted with di-acids or anhydrides, such as adipic acid, phthalic anhydride, or isophthalic acid, to produce chain-extended polyesters containing fatty acid ester side chains. Such polyesters are capable of undergoing relatively rapid cure in the presence of the aforementioned auto-oxidation catalyst systems.
Alkyd resins also may be modified through the incorporation of other polyols, i.e., diols, triols, tetraols, or higher order alcohols. In addition, transesterification may be carried out with polyols other than glycerol. One polyol of particular interest is pentaerythritol, which is a primary tetraol. This material can be esterified with fatty acids through transesterification with naturally occurring triglycerides of unsaturated fatty acids, such as those found in commercial tall oil fatty acid streams. In the latter case, pentaerythritol can be reacted with 1, 2, or 3 fatty acid moieties and incorporated into an alkyd coating system.
Another commonly used component of coating systems are uralkyds, which are analogous to the polyester alkyds. Uralkyds can be derived by substituting a di- or poly-isocyanate for a portion of the di-acid component used in alkyd preparation to achieve chain extension or grafting. Uralkyds are produced from base alkyds having excess hydroxy functionality. These polyols can be reacted at modest temperatures under catalysis with di- or poly-isocyanates to produce urethane-linked coating systems having improved mechanical, environmental, and hydrolytic performance.
Phenolic polyols are yet another useful component of coating systems. Phenolics can be "cooked" with other components, such as drying oils, or can be cold-blended with other components to produce coating systems. Phenolics are used to impart desirable characteristics, such as adhesion and corrosion resistance, to coating systems. However, phenolics are not without drawbacks. For example, phenolics typically have a high viscosity that must be reduced by using an organic solvent, thereby limiting their use in low VOC systems. Phenolics also tend to darken with age, thus changing the color of the coating. Indeed, such a color change might "bleed through" subsequently-applied coating layers, thus reducing the suitability of a primer coat comprising a phenolic moiety.
Conventional phenolics are the product of polymerization of a phenol with a formaldehyde. Two such commonly-used phenolics are p-phenylphenol/formaldehyde polymer and p-t-butylphenol/formaldehyde polymer. The former is expensive and now seldom used. The methylene linkages in the latter subject the phenolic polymer to increased risk of formation of quinone methides. Because it is the formation of quinone methides that causes the polymer to darken, p-t-butylphenol/formaldehyde polymer tends to darken and therefore is not completely satisfactory in many uses.
Improvements in color and corrosion resistance can be made by substituting some bisphenol-A for p-t-butylphenol. It is generally accepted that the isopropylidene linkage in the bisphenol-A molecule decreases the tendency for quinone methide formation in phenolic polymers. Unfortunately bisphenol-A, because of the two hydroxyl groups, has very poor solubility with oils and the common solvents used in coating formulations. Therefore, only modest modifications with bisphenol-A can be used for these polymers.
Another class of phenolic polyols has recently been developed which also is useful in coating systems. Phenolic aralkylation polyol polymers of this class exhibit improved oil solubility, an improved compatibility with oil and alkyd-based polymers, as well as with urethanes, epoxies, and acrylates and a decreased tendency for color body formation and resultant darkening of coatings in which they are incorporated. The polymers can be made substantially free of residual formaldehyde and phenol.
A lower melting polyol of this class is the phenolic aralkylation polymer reaction product obtained by aralkylating a phenolic monomer with at least one styrene derivative to obtain an aralkylated phenol, then reacting the aralkylated phenol with a coupling agent to obtain the phenol aralkylation polymer, as described in U.S. Pat. No. 5,739,259 to Hutchings et al., incorporated by reference herein. Suitable coupling agents include aryl diolefins, formaldehyde, dialdehydes, and dibenzylic diols. The aralkylated phenol is joined to the coupling agent.
A higher melting point polyol of this class is a phenolic aralkylation polymer formed by reacting a phenolic monomer with an aryl diolefin to obtain a phenol/aryl diolefin polymer. The phenol/aryl diolefin polymer then is aralkylated with at least one styrene derivative to obtain phenol aralkylation polymer.
The highly aromatic character of this class of polymers broadens the range of compatibility with other components of coating systems. Polymers of this class also exhibit enhanced physical properties, adhesion, and barrier properties. However, there exists a continuing need for improved high performance coating systems and components thereof.